1. Field of the Invention
This invention relates to the preservation of wood, and includes the chemical compositions used to treat the wood, the process of treating the wood, and the treated wood that results from the process. The chemical compositions which are described herein are especially well suited for use in the method of U.S. Pat. No. 3,968,276 in which wood to be treated is exposed to a reduced pressure for the purpose of removing moisture and volatiles from the wood, and thereafter impregnating the wood with an aqueous phenol formaldehyde/resorcinol formaldehyde chemical composition as hereinafter described.
2. Description of the Prior Art
Wood is one of the oldest materials known to man. Its ready availability in most parts of the world, coupled with the ease with which it can be cut and fabricated into useful articles, makes it well suited for a variety of applications. However, because wood is a cellulosic material, it is susceptible to decay and deterioration over any extended period of time. As a result, numerous methods have been proposed for treating wood to improve its durability, particularly when used under conditions where the wood is exposed to moisture or biological attack.
Applications of wood such as its use in manufacturing railroad ties, utility poles, fence posts, and the like, have created substantial incentives for industry to develop effective and commercially practical methods of treating the wood to extend its useful life (i.e. wood preservation). Various techniques of wood preservation have been used including surface coating, surface treating and pressure penetration of the wood with a variety of chemicals such as creosote, halogenated phenols such as pentachlorophenol and the like.
When halogenated phenols such as pentachlorophenol are used to treat wooden poles (such as utility poles), it is common to dissolve the pentachlorophenol in a hydrocarbon oil, pressure treat the poles, and then evaporate the excess oil. Such a process is relatively expensive, but is justified by the wood preservative properties of the pentachlorophenol.
However, the treated poles tend to "bleed" their treating solution into the surrounding soil when the poles are installed in the ground. This draining or leaching effect is more visible on the sunny side of utility poles which drain or bleed faster because the heat from the sun warms the treating solution and reduces its viscosity.
In an effort to reduce treating costs, water is sometimes substituted for the hydrocarbon oil, and the pentachlorophenol is used in one of its salt forms to provide water solubility. However, this approach has a number of disadvantages and is less preferred than the oil treatment.
Unfortunately, most of the present treating processes (e.g. those using creosote, various copper and mercury compounds, or pentachlorophenol), even when operated under significant pressure conditions, do not result in deep impregnation of the wood. Thus, the wood being treated with any chemical is typically treated to only a shallow depth and, in the case of logs or poles, very often the degree of penetration does not extend throughout all of the soft or sapwood, much less penetrate the heartwood.
However, U.S. Pat. No. 3,968,276 discloses a method for treating wood (logs or dimensional lumber), wherein the wood is preserved by impregnating the wood with chemicals which penetrate substantially to the core or center of the wood being treated. In the case of logs or poles, this penetration can be deep within the heartwood and may be substantially complete.
U.S. Pat No. 3,968,276 discloses enclosing the wood within a chamber, sealing the chamber, evacuating the chamber slowly so that the wood is exposed to a pressure below atmospheric pressure, introducing in to the chamber a sufficient amount of an aqueous solution of resin prepolymer to immerse the wood and then maintaining the vacuum over the immersed wood for a period of time both to allow the removal of moisture and volatiles from the wood and to permit the treating solution to enter into the wood. Thereafter, an additional amount of the resin prepolymer solution is introduced into the chamber (if needed) and the pressure on the system is increased to super atmospheric pressure to thereby cause an even deeper penetration of the treating solution into the wood. The vacuum and pressure cycle may be repeated one or more times (e.g. 1 to 30 or more cycles). When the treatment is completed to the desired extent, the treated wood is recovered from the chamber. The process described is extremely effective and achieves substantially greater penetration of the wood than any other commonly practiced treating processes.
Also it is known in the prior art, as disclosed in U.S. Pat. Nos. 4,399,195 and 4,433,031, that one or more known wood preservatives, such as the halogenated phenols, can be introduced deep into the wood being treated and obtain reduced "bleeding" or leaching, by combining the wood preservative with the method of U.S. Pat. No. 3,968,276 by using a special class of treating compositions that are water-based or water dilutable which permits the benefits of the aqueous resin prepolymer system of U.S. Pat. No. 3,968,276 to be maintained.
Further, U.S. Pat. Nos. 4,399,195 and 4,433,031 disclose that it is possible to accomplish deep penetration with reduced bleeding without encountering any significant amount of "blooming" which is normally associated with aqueous treating solutions of halogenated phenols. This blooming effect is caused by the fact that the aqueous treating solutions are made from salts of halogenated phenols, which dissolve readily in water. Because the salts are water soluble, there is a tendency for the salts to be redissolved by rain water or ground water and to migrate easily to the surface of the treated wood where they crystallize during the subsequent evaporation of the water, causing the "blooming effect".
U.S. Pat. Nos. 4,399,195 and 4,433,031 disclose manufacturing a water dilutable concentrate or a water-based treating solution which consists essentially of: (a) a halogenated phenol wood preservative such as pentachlorophenol; (b) a phenolic resin prepolymer such as phenol formaldehyde prepolymer; (c) a coupling or solubilizing agent for enhancing the water compatibility of the concentrate (e.g. methyl alcohol or acetone with or without the addition of a surfactant); and (d) water. Optionally and preferably, the disclosed treating solutions contain organo/metallic salts, especially copper or zinc salts.
In addition, U.S. Pat. Nos. 4,339,195 and 4,433,031 disclose that the ability of the prepolymer to cure rapidly and lock the halogenated phenol deep within the wood can be enhanced by heating the wood at the conclusion of the treatment (e.g. heating the wood with steam) to thereby accelerate the curing of the resin prepolymer.
The term "curing" as used in the previous paragraph is to be differentiated from the term "gelling" as used hereinafter in the present art. Curing refers to the process by which wood preservative stabilizes once impregnated into the wood. Gelling refers to the solidification or semi-solidification of the resins forming the wood preservative during preparation of the wood preservative. Gelling or, more specifically, the time it takes for gelling to occur (gel time testing) is used in the art as an indicator of resin activity and is used to predict and select curing parameters and characteristics of the resins used in the wood preservative solutions.
While the process for treating wood as disclosed in U.S. Pat. Nos. 4,399,195 and 4,433,031 works quite well, the wood treating compositions of the prior art suffer from the limitations of having a limited shelf life, relatively slow curing times and the requirement of significant amounts of heat to effect curing.
It was also known in the prior art to use water soluble phenol formaldehyde resoles as wood preservatives. Such a resole of the prior art typically gels in approximately 10 to 20 minutes at 121.degree. C. and has a shelf life of 3 to 6 weeks at 25.degree. C. (measured by its ability to remain infinitely water dilutable).
In the adhesive art (glues, etc., an art distinctly different from the wood preservative art), it was known that resorcinol formaldehyde resins could be added to or blended with certain water soluble phenol formaldehyde resoles which contain free formaldehyde to react with the resorcinol, wherein the resorcinol functioned to cause more rapid gelling and curing of the adhesive at room temperature. However, the resorcinol formaldehyde modified phenol formaldehyde resins of the adhesive arts were not used as wood preservatives and invariably exhibited a much shorter shelf life than non-blended resins. Further, the resorcinol modified resins of the adhesive arts were designed to fully cure at room temperature, which is not compatible with the wood preservative arts, where the resin must be retained in a liquid state for extended periods both on the shelf and during the impregnation process. While curing at room temperature after the impregnation process is complete may be compatible with the wood preservative arts, premature gelling of the resin solution at room temperature is not. Gelling of the resins in the wood preservative arts is to be avoided because a fully gelled resin cannot impregnate the wood at all, and the large molecules in a partially gelled resin cannot effectively impregnate the minute interstices of the wood and, therefore, cannot penetrate deep into the heartwood, either.
It would be preferred if the known wood preservative resins of the prior art could cure faster and at lower temperatures after impregnation than those presently known in the art, exhibit extended shelf life but without premature gelling. Also, while free or unreacted formaldehyde may be necessary with phenol formaldehyde resins of the prior art for reaction purposes, it would also be preferred if such resins would exhibit an extremely low free formaldehyde content during resin formation, during the impregnation process at the wood impregnation plant, and after the wood is cured and used in its final application, because formaldehyde is toxic and is considered an environmental hazard. Further, it would be preferred if such resins exhibited increased tolerance to the additives which are normally added to these resins to aid in wood preservation and fire retardance.